A search for distant radio galaxies from SUMSS and NVSS -- I. Sample definition, radio and K-band imaging

Abstract

We present the first results from a pilot study to search for distant radio galaxies in the southern hemisphere (δ 15 mJy. We observed 71 sources without bright optical or near-infrared counterparts at 1.385 GHz with the ATCA, providing ∼5 arcsec resolution images and subarcsec positional accuracy. To identify their host galaxies, we obtained near-infrared K-band images with IRIS2 at the AAT and Soft at the NTT. We identify 92 per cent of the USS sources down to K ∼ 20.5. The SUMSS-NVSS USS sources have a surface density more than 4 times higher than USS sources selected at lower frequencies. This is due to the higher effective selection frequency, and the well-matched resolutions of both surveys constructed using the same source fitting algorithm. The scattering of α > -1.3 sources into the USS sample due to spectral index uncertainties can account for only 35 per cent of the observed USS sources. Because our sample appears to contain a similar fraction of very distant (z > 3) galaxies, selecting USS sources from SUMSS-NVSS should allow us to identify large numbers of massive galaxies at high redshift.